This isn't true at all. Leftism is inherently anti-war. Hence, "no war but class war".
The problem is where the anti-war stance is misrepresented. Unless it is coming from someone inside of Russia (and possibly countries positioning themselves as allied to Russia), who has a chance of possibly affecting Russia's ability to carry out the war through direct action and other political involvement, the stance that the solution is for Russia to back down is not an anti-war position; it is the exact opposite; it is the pushing of the "de-escalation through escalation" bullshit. It is a chauvinistic "our side will never back down" death pact.
The stance of people in Ukraine, the U.S., any NATO country—anywhere that potentially has influence over the Empire, really—must be to push Ukraine to back down in order to be authentically anti-war.
There's also the more controversial leftist argument over whether states can be authentically anti-imperialist. But that's really going to be a matter of whether you support the position I alluded to above for people who have potential political influence over nations which some people would consider "anti-imperialist nations" (due to the intermeshing of imperialism and class conflict, and also the fact that the Empire inherently has more power over, and responsibility toward, the prosecution and ending of wars than its targets). Like, that more controversial position comes up in the rarely seem question of whether Russian leftists should be cheering on and supporting the Russian military in nationalistic zeal, or trying to get Russia to negotiate and back down with as much fervor as Western leftists try to do so with U.S./NATO/Ukraine. We should all be able to agree, at the very least, that those who have the slightest chance at influencing the Empire to back down should do so. And that 100% includes Hasan Piker, who lives, acts, and practices social punditry within the U.S.
I'm pretty sure it's just called "NASA", actually.
I mean, communism would be even better, of course. But doing better than SpaceX is nothing particularly revolutionary in itself.